Reactive ion or plasma etching processes have been widely used for etching compound semiconductors, particularly III-V compounds, in the fabrication of optoelectronic devices such as lasers. A particular requirement of any fabrication process in this field is that of providing high quality flat optical surfaces e.g. mirror facets. Generally a methane/hydrogen mixture is employed as the etchant gas, but it has been found that this composition has a number of major disadvantages. Firstly, when etching a multilayer or heterostructure it is extremely difficult to provide smooth correctly oriented etched surfaces which can act as low loss mirrors. Secondly, polymer deposition can take place on the masking material and on the etched surface leading to surface roughening. Thirdly, the orientation of the etched surface is critically dependent on the balance between polymer deposition and etching of the semiconductor. Small variations in the etch conditions can thus have a significant effect on the quality of the etched surface. In consequence the process is difficult to control and results in relatively low yields of acceptable devices.
In an attempt to address these disadvantages some workers have investigated the addition of free oxygen to the etchant gas mixture. A process of this nature is described by J W McNabb et all in J. Vac. Sci. Technol., B9(6), Nov/Dec 1991, p.3535. This has been found to reduce the problem of polymer deposition, but has shown little improvement in the reduction of surface roughness and the control of surface orientation. It will be appreciated that any roughening of a reflective surface causes significant loss of optical signals reflected from that surface.
The object of the present invention is to minimise or to overcome these disadvantages.